In recent years, development of display devices has rapidly progressed, and various display devices such as a liquid crystal display device and an EL display device have been extensively used for office automation equipment such as a personal computer and a word processor. These display devices have a sandwich structure in which a display element is disposed between transparent conductive films.
A silicon-based semiconductor film is mainly used for a switching element that drives a display device. This is because a silicon-based thin film exhibits excellent stability, excellent workability, a high switching speed, and the like. A silicon-based thin film is generally formed by chemical vapor deposition (CVD).
An amorphous silicon-based thin film has a problem in that a video or the like cannot be displayed at a high frame rate due to a relatively low switching speed. A crystalline silicon-based thin film achieves a relatively high switching speed, but requires a high temperature of 800° C. or more, laser heating, or the like for crystallization. Specifically, a large amount of energy and a large number of steps are required for producing a crystalline silicon-based thin film. A silicon-based thin film exhibits excellent performance as a voltage element, but shows a change in characteristics with time when causing a current to flow therethrough.
Therefore, use of a film other than a silicon-based thin film has been studied.
A transparent semiconductor thin film formed of indium oxide, gallium oxide, and zinc oxide, and a transparent semiconductor thin film formed of zinc oxide and magnesium oxide have been proposed as a transparent semiconductor film that exhibits excellent stability as compared with a silicon-based thin film, and has a light transmittance equal to that of an ITO film, and as a target used to produce such a transparent semiconductor film (e.g., Patent Document 1).
Patent Document 2 discloses a composition that includes indium oxide and gallium oxide as the main components, and contains an InGaO3 compound. Patent Document 2 relates to a transparent conductive film.
Patent Document 3 discloses an In2O3 transparent conductive film that contains 1 to 10 atom % of gallium (Ga). The transparent conductive film disclosed in Patent Document 3 is produced by cosputtering in a state in which gallium metal is disposed on an In2O3 target.
Patent Document 4 discloses a sintered body composition for a transparent conductive film that has a Ga/(In+Ga) ratio of 0.35 or more and less than 1.0, and includes a (Ga,In)2O3 phase. Patent Document 5 discloses an oxide sintered body for a transparent conductive film that has a Ga/(In+Ga) ratio of 0.65 or more and less than 1.0, and includes a (Ga,In)2O3 phase.
Patent Document 6 discloses an indium oxide-gallium oxide sintered body that contains 49.1 to 65 atom % of gallium. Patent Document 7 discloses a sputtering target using a sintered body having a Ga/In ratio of 0.97 to 1.86. Patent Document 8 discloses an indium oxide-gallium oxide sintered body that contains 35 to 45 atom % of gallium, and Patent Document 9 discloses an indium oxide-gallium oxide sintered body that contains 15 to 49 atom % of gallium.
However, a crystalline oxide semiconductor film formed of indium oxide and gallium oxide cannot be obtained when the gallium content is within the ranges disclosed in Patent Documents 6 to 9. Moreover, when using the compositions disclosed in Patent Documents 6 to 9, an InGaO3 crystal phase that exhibits high insulating properties may be produced, so that an abnormal discharge or nodules may occur during sputtering.
An object of the invention is to provide a non-silicon-based semiconductor thin film that may be used for a thin film transistor, and a sputtering target used to produce the non-silicon-based semiconductor thin film. Another object of the invention is to provide a thin film transistor that includes a novel non-silicon-based semiconductor thin film.